which is based on the commutation algebra for the \({\cal A}_{...}\) -, \({\cal B}_{...}\) - and \({\cal Q}_...\) -operators many times used in previous works.[1]}, [2]}, [3]}, [4]} The matrix elements, where bra- and ket- vectors are multi-exciton states, have also been numerously calculated (see, e.g., Ref. di19). In this letter we omit the details of relevant algebraic manipulations and present only the main key points of this procedure. First, note that the sum \({\cal A}_{\bf q}^\dag \!+\!{\cal B}_{\bf q}^\dag \)  commutes with \({\cal Q}_{\bf 0}^\dag \) , hence, the only term of operator (REF ) contributing to expectation (REF ) is \(-{\cal N}_\phi (q^2\!/2)e^{-q^2\!/4}{\cal B}_{\bf q}^\dag \) . By calculating
